Configurable hybrid medium access control for cable metropolitan area networks

ABSTRACT

The hybrid medium access control MAC system is configurable to the type of communication required to support a desired application or service. The MAC system analyzes services requested from a settop terminal, or a network controller client, and determines the best MAC component for transmitting signals upstream based on the resources required by the service and the available network resources. A portion of the upstream channel spectrum is allocated to each of the MAC components: 1) POP; 2) assigned static TDMA; 3) assigned dynamic multi-rate TDMA; and 4) random slot reservation--dynamic slot allocation TDMA. Depending upon the communication requirements of the service desired, the configurable MAC system will select the MAC component best suited to support the service. A frequency agile transmitter is then tuned to the channel which has been preallocated for the selected MAC component(s). As communication traffic varies over time, the system reallocates portions of the upstream channel spectrum among the different MAC components and may also reconfigure a specific MAC component.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part to application Ser. No.08/395,325 filed Feb. 28, 1995, entitled DISTRIBUTED SYSTEM ARCHITECTUREFOR DIGITAL BROADCAST AND INTERACTIVE SERVICES to Reem Safadi. Thisapplication is also related to co-pending application Ser. No.08/402,027 filed concurrently herewith, entitled ADAPTIVE PROTOCOLCOMMUNICATION SYSTEM to Reem Safadi which is incorporated by referenceas if fully set forth herein.

BACKGROUND OF THE INVENTION

1. Field Of The Invention

This invention relates generally to cable television communicationsystems. More particularly, the invention relates to a cable televisioncommunication system having a configurable hybrid medium access controlsystem which facilitates efficient use of the upstream bandwidth inbi-directional cable television systems and equitable upstream channelaccess for communicating entities.

2. Description Of Related Art

Cable television (CATV) communication systems have traditionallycomprised uni-directional systems which primarily provide videoprogramming services to individual homes. These systems only permitcommunications in a downstream direction from the headend of a CATVsystem to the plurality of settop terminals in individual homes.However, bi-directional CATV communication systems have becomeincreasingly standard, and almost necessary, as the popularity anddiversity of interactive services has grown, such as pay per view(current) and interactive banking and home shopping (near future).Bi-directional CATV systems support both downstream and upstreamcommunication. Individual subscribers, through the use of a settopterminal coupled to a television, may communicate with the headend,other subscribers or service providers within the system. These systemsalso permit subscribers to select specific video programming or consumerservices and pay only for those services which are used. Other servicesrequiring the use of upstream communication channels include medical,fire and burglar alarm services, subscriber polling and telemetry suchas utility meter reading.

To facilitate a bi-directional communication flow, the frequencyspectrum of the cable is divided into a downstream path originating atthe headend and an upstream path originating at the settop terminals. Inorder to effectively utilize the upstream bandwidth, bi-directional CATVcommunication systems require medium access control (MAC) forcoordinating the upstream transmissions from various settop terminals.Without such coordination, simultaneous upstream transmissions from thesettop terminals will cause the signals to collide, resulting in anunintelligible signal and loss of data from all transmissions.

Traditional medium access control methods include plain old polling(POP), random access as in ALOHA, static time division multiple access,dynamic time division multiple access and multiple access with collisiondetection (MACD). These are examples of MAC in satellite channels orother systems with shared media. Each MAC method has throughput andaccess delay characteristics associated with the method employed.

Plain old polling (POP) is implemented by a network controller withinthe CATV headend which communicates with a plurality of settopterminals. When a settop terminal requires a service, a message or flagis placed in the transmit queue within the settop terminal. A pollingcycle is initiated periodically by the network controller to empty thetransmit queue of each settop terminal. The network controllerinterrogates in succession every settop terminal on the sharedcommunication medium served by the network controller to determine whichof the terminals are in need of services. This method is intended forapplications that benefit from a store and forward system, wherecollisions in the upstream bandwidth are expected but latenciesassociated with the POP response delivery are irrelevant. In addition,it facilitates controlled communications for any diagnostic operation.

The disadvantage with POP is that it exhibits poor performance in termsof network efficiency, especially when a small subset of the settopterminals require access to the channel during a polling cycle. Further,the method is not suitable for connection-orientated services whichrequire guaranteed bandwidth since the settop terminal may be subject tovariable latencies in upstream channel access. This type of MAC is leastsuitable for interactive services.

In time division multiple access (TDMA), an upstream carrier frequencyis formatted as a tightly synchronized series of repeating frames, eachframe being divided into a number of time slots. Each time slot carriescommunications pertaining to a single settop terminal. Since thesynchronization of the time frames is critical, the communicatingentities, including the headend and all settop terminals, must besynchronized.

TDMA may be either static, dynamic or multiple access with collisiondetection. In the static (or fixed cycle) TDMA method, each settopterminal has a pre-allocated transmission time slot during which it mayaccess the upstream channel. In this case, the CATV communication systemis only capable of supporting the same number of settop terminals as thenumber of time slots, if each settop terminal is permitted to reserveonly a single time slot per frame. Time slots must be used as assignedto prevent any transmission overlap from different settop terminals.This method has the advantage of providing guaranteed latency per cycle.

The disadvantage with static TDMA is that the cycle time, the durationbetween frames, is bound to be longer than necessary since each settopterminal is pre-assigned a time slot whether it needs the time slot ornot. If on the average only ten per cent of the terminals communicate atany given time, then ninety per cent of the available channel capacityis wasted. Since the method is static, the cycle duration cannot improveover that which presently exists (slot-time × total number ofterminals).

In the dynamic (variable cycle) TDMA method, time slots are allocated asdemanded by individual settop terminals. A random time slot is reservedby the settop terminal that wishes to initiate communications. Thenetwork controller dynamically allocates the available time slots asrequests are received from settop terminals. The dynamic allocation oftime slots optimizes the use of the available bandwidth. However, thedisadvantage with dynamic TDMA is that settop terminals may be forced towait for a time slot when multiple settop terminals are competing for afewer number of available time slots. Thus, the settop terminalsexperience a delay prior to the allocation of a time slot by the networkcontroller. There is a point of diminishing returns for dynamic TDMAwhen the average access delay due to multiple terminals attempting toreserve the same time slot becomes greater than the fixed TDMA cycleduration.

Carrier sense multiple access with collision detection (CSMA/CD) is arandom access mechanism with no knowledge of the order of transmissions(this is the access scheme used in an Ethernet local area network). Thecarrier-sense is impractical in existing CATV communication systems andis generally not employed. Although the state of the upstream channelcan be inferred from the downstream channel by providing feedbackinformation on the downstream channel, the settop terminals cannotaccurately monitor the upstream channel due to the physical limitationsof most existing CATV communication systems. In the tree and branchtopology of a CATV system, an upstream transmission on one feeder cablecannot be received on the upstream channel of another feeder channel.Further, drop cable directional taps, the taps that connect the dropcables to the feeder cable, inhibit the detection of the transmission ofanother settop terminal even if it is on the same feeder.

Although CSMA/CD facilitates allocation of bandwidth on demand, it fallsshort of providing a guaranteed bandwidth to a settop terminal. Carriersense exhibits unnecessary access delays as the number of activeterminals reach a threshold beyond which throughput experiencesintolerable delays. Therefore, it cannot efficiently supportconnection-orientated services and applications. Further, variablelatencies may require communication traffic modelling for properresource allocation in addition to imposing undesired limitations onsettop terminal distance from the headend.

Accordingly, there exists a need for a simple medium access controlmethod which efficiently utilizes the available upstream bandwidth andallocates the bandwidth required to support communications for requestedapplications and services with various traffic characteristics, such aslatency, bandwidth and throughput requirements.

SUMMARY OF THE INVENTION

The hybrid medium access control system of the present invention isconfigurable to the type of communication required to support thedesired application or service. The MAC system analyzes servicesrequested from a settop terminal (or a network controller client) anddetermines the best MAC component for transmitting signals upstreambased on the resources required by the service and the available networkresources.

A portion of the upstream channel spectrum is allocated to each of theMAC components: 1) POP; 2) assigned static TDMA; 3) assigned dynamicmulti-rate TDMA; and 4) random slot reservation--dynamic slot allocationTDMA. Depending upon the communication requirements of the servicedesired, the configurable MAC system will select the MAC component bestsuited to support the service. A frequency agile transmitter is thentuned to the channel which has been preallocated for the selected MACcomponent. As communication traffic varies over time, the systemreallocates portions of the upstream channel spectrum among thedifferent MAC components and may also reconfigure a specific MACcomponent.

The configurability feature of the hybrid MAC ensures that there are norestrictions on the type of applications or services that aretransmitted over the CATV network, provided that the physicallimitations, such as available return spectrum bandwidth, have not beenreached. The network administrator is free to choose among anycollection of such services and have the hybrid MAC dynamicallyconfigured by the network controller. One example of a dynamicconfiguration is when the frame size and time slot size within a cycleare both determined by the required latency and throughout per node toefficiently support a particular application. In this manner trafficloading assumptions need not be imposed.

This approach relieves the CATV network administrator from having toknow the traffic pattern associated with the usage of such services,especially since many of these services have not yet been established orfully characterized. Further, the hardware within the settop terminalneed not be specified to always support the most demanding application.This simplifies the settop terminal, thereby providing a lower costimplementation of the system.

Various services have different bandwidth and latency requirements. Theextent to which these services require support will inevitably varydepending on the success of that specific service. Success of aparticular service depends on many factors, such as the demographics ofa certain geographic area, socio-economic trends and service appeal ingeneral. The configurable aspect of the hybrid MAC provides uniqueflexibility in both MAC implementation and utilization by the networkadministrator.

Accordingly, it is an object of the present invention to provide ahybrid MAC system which optimizes the resources of a CATV communicationnetwork depending upon the applications and services requested.

It is a further object of the invention to provide a hybrid MAC systemthat is configurable to the demands placed upon the communicationnetwork at any given time.

It is a further object of the invention to provide a hybrid MAC systemwhich operates in the space, frequency and time domains.

Other objects and advantages of the system will become apparent to thoseskilled in the art after reading the detailed description of a presentlypreferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an end-to-end cable televisioncommunication network embodying the present invention;

FIG. 2 is a block diagram of the remote/local hub of the presentinvention;

FIG. 3 is a block diagram of a headend/central office of the presentinvention;

FIG. 4 is a block diagram of the cable television distribution networkof the present invention;

FIG. 5 is a block diagram of a settop terminal of the present invention;

FIG. 6 is a time division multiple access frame utilized in thepreferred embodiment of the hybrid MAC system;

FIG. 7 is a block diagram of the space, frequency and time domains ofthe hybrid MAC system;

FIG. 8 is a flow diagram of the random slot reservation/dynamic slotallocation time division multiple access component of the hybrid MACsystem;

FIG. 9 is a flow diagram of the multi-rate dynamic time divisionmultiple access component of the hybrid MAC system; and

FIG. 10 is a block diagram of a network controller which implementsmedium access control.

TABLE OF ACRONYMS

AAL5 ATM Adaption Layer 5

AC Addressable Controller

ASEM Access Subnetwork Element Manager

ATM Asynchronous Transfer Mode

APP Adaptive Protocol Processor

CLP Cell Loss Priority (ATM)

CRC Cyclic Redundancy Code

DLL Data Link Layer

DSA Dynamic Slot Allocation

DTE Data Terminal Equipment

ECM Entitlement Control Messages

EIA Electronics International Association

EMM Entitlement Management Messages

FDM Frequency Division Multiplexing

FTTN Fiber To The Node

GFC Generic Flow Control (ATM)

HEC Head Error Check (ATM)

HFC Hybrid Fiber Coax

IBTM In-band Transport Multiplex

IP Internet Protocol

IR Infrared

IPPV Impulse Pay Per View

ITEM Integrated Transport Encryption Multiplexer

LAN Local Area Network

LLC Logical Link Control

L1G Level One Gateway (regulated by the FCC)

L2G Level Two Gateway (unregulated by the FCC)

MAC Medium Access Control

MPEG2 Motion Picture Expert Group 2

MPTS Multi Program Transport Multiplex (MPEG2)

NVRAM Non-Volatile Random Access Memory

PES Packetized Elementary Stream

PSI Program Specific Information

OAM&P Operation Administration Maintenance And Provisioning

OBTM Out Of Band Transport Multiplex

OSI Open Systems Interconnection

OSS Operation Support System

PAT Program Association Table (MPEG2)

PCR Program Clock Reference (MPEG2)

PCS Personal Communication Services

PDU Protocol Data Unit

PID Packet Identifier

PMT Program Map Table

POP Plain Old Polling

POTS Plain Old Telephony Service

PPV Pay Per View

PSI Program Specific Information

PSP Protocol Syntax Processor

PT Payload Type

QAM Quadrature Amplitude Modulation

QPSK Quadrature Phase Shift Keying

RSR Random Slot Reservation

SDU Service Data Unit

SPTM Single Program Transport Multiplex (MPEG2)

STT Settop Terminal

VCC Virtual Channel Connection (ATM)

VCI Virtual Channel Identifier (ATM)

VDT Video Dial Tone

VIP Video Information Provider (information owner)

VIU Video Information User (subscriber)

VPI Virtual Path Identifier (ATM)

WAN Wide Area Network

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A CATV communication network 10 embodying the present invention is shownin FIG. 1. The communication network 10 generally comprises aremote/local hub 14 which communicates with a plurality ofheadends/central offices 18, each of which in turn communicates with aplurality of settop terminals (STTs) 16. The STTs 16 are the interfacebetween the television of a video information user (VIU) and thecommunication network 10. The remote/local hub 14 may be physicallylocated remote from the headends 18 or, alternatively, may be located atthe site of any one of the headends 18. The communication network 10interfaces with a plurality of video information providers (VIPs) 12which provide compressed digital video and services. Through the remotehub 14 and the headends 18, the communication network 10 providestwo-way transparent (protocol stack independence, layer 3-7) datatransport service between the VIPs 12 and the video information users atthe STTs 16. The hub 14 provides broadcast information services from theVIPs 12 to all STTs 16 on the network 10. The headends 18 facilitateinteractive communications between the VIPs 12 and the STTs 16 that areserved by that particular headend 18. In the preferred embodiment of theinvention, communications between the VIPs 12, the remote/local hub 14and the headend/central offices 18 are transmitted over a fiber opticmedium.

To provide the bi-directional communication flow over the network 10,the frequency spectrum of the physical medium from the headend 18 to theSTTs 16 is divided into a downstream signal path originating at theheadend 18 and an upstream signal path originating at the STTs 16. Thebandwidth of the physical medium in the preferred embodiment extends upto 1 GHz. The downstream bandwidth typically employs frequencies above50 MHz, and the upstream frequencies below 50 MHz. The downstreambandwidth is divided into 6 MHz QAM channels. The channels in theupstream bandwidth are 192 KHz QPSK channels. In the present invention,a portion of the channels are allocated for analog communications andthe remainder for digital communications. Accordingly, analog anddigital communications may be frequency division multiplexed (FDM) overthe separate channels and transported over the same physical medium.Analog CATV communication systems are well known in the art, such as thesystem disclosed in U.S. Pat. No. 4,533,948, (to McNamara et al.) and inU.S. Pat. No. 4,245,245, (to Matsomoto et al.), which are incorporatedby reference as if fully set forth.

Referring to FIG. 2, a remote/local hub 14 made in accordance with theteachings of the present invention is shown. The hub 14 includes a level1 gateway (L1G) 20, an access subnetwork element manager (ASEM) 22, anaddressable controller (AC) 24, a billing system 26 (co-located orremotely located), an operations support system (OSS) 28 (co-located orremotely located), an integrated transport encryption multiplexer (ITEM)30, a 64 quadrature amplitude modulation (QAM) modulator 32, an RFupconverter 34, a quadrature phase shift keying (QPSK) modulator 36(optional) and an asynchronous transfer mode (ATM) services multiplexer38. A backbone subnetwork 40 provides the physical medium and theoptical to electrical interface for components within the hub 14 tocommunicate with each other and for butside entities (such as the VIPs12 and STTs 16) to communicate with, and through, the hub 14.Communications between the hub 14 and the headend 18 elements areconducted via Internet protocol, asynchronous transfer mode (IP/ATM)signaling in WAN connectivity and IP/Ethernet in LAN connectivity. Theremay be more than one of each of these components depending upon systemcapacities (i.e. desired number of channels for the ITEM 30, number ofsubscribers for the AC 24 etc.).

The specific components which comprise the network architecture of thepresent invention will now be presented in detail. The video informationprovider (VIP) 12 consists of a level two gateway (L2G) and associatedservers. The L2G acts as an interface between the VIP 12 and the network10. The VIPs 12 are the source of live, archival broadcast orinteractive service content, (comprising electronic encyclopedias,electronic catalogs, downloadable applications, movies, etc.),communications with the network 10, and service menus. The L2Gcommunicates with the L1G 20 to manage the establishment and terminationof service/session connections between the VIUs and the VIPs 12. The L2Galso provides menu presentation and selection processing to and from theVIUs, performs VIU authentication and forwards billing information, forVIP 12 provided services to the billing system 26.

The level 1 gateway (L1G) 20 provides overall management of network 10in support of service delivery from a VIP 12 to the VIUs. The L1G 20performs the following functions: 1) management of VIP-VIU interactivesessions through communication with the VIPs 12; 2) VIU authenticationand collection of billing information relating to network 10 support ofVIP 12/L1G 20 provided services for forwarding to the billing system 26;3) interactive menu presentation which is responsive to the serviceselected; and 4) database management of VIU profiles for digitalbroadcast and interactive services.

The access subnetwork element manager (ASEM) 22 acts as an agent to(i.e. is controlled by) the L1G 20 and/or OSS 28 (depending on thefunctions to be performed by the ASEM 22). At the direction of the L1G20, the ASEM 22 provides management of headend 18 components (shown inFIG. 3), much in the same manner as the L1G 20 oversees management ofresources on the backbone subnetwork 40 (via a designated backbonenetwork manager not shown). The ASEM 22 determines which ITEM 50 andnetwork controller 62 can accommodate a new connection (based on alreadyutilized resources within each headend 18) and the L1G 20. The ASEM 22conveys parameters such as ATM virtual path identifier (VPI) and virtualchannel identifier (VCI) values for session signaling and sessioncontent to the ITEMs 30, 50 and the network controllers 62. The ASEM 22also conveys associated transmission rates for the downstream ATMVPI/VCI and signaling rates for upstream as conveyed to it by the L1G 20(these values are originated by the VIP 12/L2G). The ASEM 22 forwardsappropriate scheduling parameters to the addressable controller 24 forencryption of pay-per-view (PPV) and impulse-pay-per-view (IPPV)services by the ITEMs 50. In the preferred embodiment of the invention,the ASEM 22 oversees OAM&P (Operation Administration Maintenance andProvisioning) functions through the backbone subnetwork 40 andinterfaces with the OSS 28 to convey status or any OAM&P information ofelements (Mux/Mods, Demod/Muxes, etc.) to the OSS 28.

The addressable controller 24 manages the secure delivery of broadcast,pay-per-view and non-video-on-demand (staggercast) services, includingVIU authorization for those services and program scheduling bycontrolling the encryption subsystem. Access control and encryptionparameters are forwarded to network elements which perform downstreamand upstream encryption and decryption. In the preferred embodiment ofthe invention, downstream encryption is implemented in the ITEMs 30, 50and downstream decryption is implemented in network modules 70, whichare part of each STT 16. Upstream encryption is implemented in thenetwork module 70 and upstream decryption is performed by a networkcontroller 62. For interactive service communications, which arefacilitated at the headends 18, the addressable controller 24preprovisions the ITEMs 50 and the network modules 70 with theappropriate encryption/decryption parameters. For broadcast servicecommunications, which are facilitated by the hub 14, the addressablecontroller 24 forwards similar parameters to the ITEMs 30 based onscheduling information forwarded by the L1G 20 through the ASEM 22.

The integrated transport encryption multiplexer (ITEM) 30 providessecure delivery of broadcast digital services information to the VIUs asan in-band transport multiplex (IBTM). The ITEM 30 originates the IBTMincluding video, audio and data by performing ATM to MPEG2 reassemblyand re-adaption (AAL5) of single program transport multiplexes (SPTM).This includes ATM to MPEG2reassembly of audio-visual content, ATM toAAL5-service data units (SDU) reassembly of non-MPEG2 data and removingjitter and adjusting program clock reference (PCR) timing for audiovisual content. The ITEM 30 creates an aggregate MPEG2 multiplex fromany input SPTM to any output multi-program transport multiplex (MPTM).In doing so, the ITEM 30 uniquely reassigns packet identifier (PID)values, creates an aggregate stream (program specific information (PSI)including a program association table (PAT) and a program (PMT) maptable and selectively encrypts SPTMs as instructed by the ASEM 22. TheITEM 30 updates the aggregate stream PCR and inserts entitlement controlmessages (ECMs). It also performs similar operations when multiplexingL1G 20 signaling and addressable controller 24 messages (includingentitlement management message (EMMs)) on the out-of-band transportmultiplex (OBTM) which is then forwarded to the QPSK modulator 36.

The ITEM 30 provides a transport interface for digital broadcastcommunications between the VIPs 12 and the STT 16 via the backbonesubnetwork 40 (or through direct interfaces if co-located). Broadcastaudiovisual information and signaling originated by the VIPs 12 isforwarded to the STT 16 via the ITEM 30. More specifically, the ITEM 30accepts an ATM stream via an ATM/SONET interface (with sustained cellrate version of AAL5) and selects the appropriate cells for MPEG2 packetreassembly based on the value of the VCI which is conveyed by the ASEM22 to the ITEM 30 during connection establishment. This is achievablesince the virtual connection between the VIP 12 and the VIU ismaintained end-to-end. The resulting MPEG2 transport multiplex(consisting of multiple audiovisual information streams,protocol-independent, from an ITEM 30 standpoint, information streamscarried as AAL5-SDUs (such as stock quotes) is output to the 64 QAMmodulator 32.

In order to ensure secure delivery of a given broadcast audiovisualservice, the ITEM 30 selectively encrypts, pursuant to the addressablecontroller 24 configuration, a given set of MPEG2program streams withinthe MPEG2 transport stream. Access control and encryption relatedinformation is forwarded to ITEM 30 from the addressable controller 24.The ITEM 30 incorporates this information within the MPEG2 transportstream (per MPEG2 rules where applicable) and encrypts as instructed bythe addressable controller 24. Each output of the ITEM 30 is forwardedto a single 64 QAM modulator 32 whose output is converted to theselected downstream channel by an RF upconverter 34 and then combined byan RF combiner 35 for downstream transmission. Each ITEM 30 keeps trackof the resources required for a given service.

The QAM modulator 32 accepts the downstream IBTM output of ITEM 30,applies forward error correction and 64 QAM encoding. The output of theQAM modulator 32 is input to the RF upconverter 34.

Downstream OBTM signaling intended for all subscribers from the L1G 20or the addressable controller 24, (and optional application downloadsfrom the L1G 20, or L1G 20 menus to VIPs 12), are sent from the ITEM 30to the QPSK modulator 36. The QPSK modulator 36 accepts the output ofthe ITEM 30. The QPSK modulator 36 also provides forward errorcorrection and QPSK modulation for the OBTM transmission. The output isinput to the RF combiner 35 for downstream transmission.

The RF upconverter 34 receives the outputs from the QAM modulator 32 forIBTM (and the QPSK modulator 36 for OBTM if upconversion was notperformed by the QPSK modulator 36). Each RF upconverter 34 is capableof accepting the output from two modulators 32, 36. The RF upconverter34 accepts two intermediate carrier frequencies (41.0 -47.0) with anassociated RF output which is frequency agile from 50 to 1000 MHz. Theupconverter 34 will accept either the QAM modulator output 32 at IF fromthe ITEM 30 or an analog television channel input (not shown) at IF. Theanalog inputs will be upconverted to predetermined analog channels. TheRF upconverter 34 is controlled either via front panel switches orremotely via the Ethernet interface. The output from the RF upconverter34 is input to an RF combiner 35.

The RF combiner 35 is a full bandwidth 5 to 1000 MHz 12-waycombiner/splitter. It may be used to merge signals from up to twelvechannels or to distribute a single signal to twelve outputs. The RFcombiner 35 employs directional coupler circuitry to attain high channelisolation for protection from channel to channel interference. Theoutput from the combiner 35 bypasses the processing within the headend18 and proceeds directly to the headend combiners (not shown) forfurther combination with the output from the headend 18.

The asynchronous transfer mode (ATM) services multiplexer 38 aggregatesthe non-null ATM cells from each DS3 interface (up to n DS3 interfaces,where n ranges between 1 and 18 per ATM services mux 38) and forwardsthe multiplexed cells to the backbone network 40. It also acts as an ATMedge device for the OAM&P in the headend 18 providing virtual LANconnectivity between the hub 14 and the headends 18. The backbonesubnetwork 40 interconnects the LAN 39 where the ASEM 22 and theaddressable controller 24 reside to the LAN 61 within the headend 18 toappear as though all components reside on the same LAN.

The operation support system 28 provides the OAM&P services in supportof the network 10. The billing system 26 stores the billing informationrelated to each VIU and generates billing statements. The generaloperation and functions of both of these components is well known tothose skilled in the art.

Referring to FIG. 3, a headend 18 made in accordance with the teachingsof the present invention is shown. The headend 18 facilitates digitalinteractive communications between the VIPs 12 and the VIUs. All of thedigital broadcast communications which were processed by the hub 14 arefurther combined at the output of the headend 18 by combiners (notshown). Accordingly, no further processing of the broadcastcommunications by the headend 18 is required.

The headend 18 includes an ITEM 50, a 64 QAM multiplexer/modulator(mux/mod) 52, an RF upconverter 54, a QPSK multiplexer/modulator(mux/mod) 58, a QPSK demodulator/multiplexer (demod/mux) 60, a networkcontroller 62 and a headend ATM services mux 64. The headend 18communicates with a plurality of STTs 16 through the CATV transmissionnetwork 56. Each STT 16 includes a network module 70 for interfacingwith the CATV transmission network 56.

The ITEM 50 provides secure delivery of interactive digital servicesinformation to the VIUs as an IBTM. Additionally, it can be configuredto provide broadest services to subscribers served by that particularheadend 18. The ITEM 50 originates the IBTM including video, audio anddata and VIP 12 signaling by performing ATM to MPEG2 reassembly andre-adaption (AAL5) of SPTMs. This includes ATM to MPEG2 reassembly ofaudio-visual content, ATM to AAL5-SDU's reassembly of session signalingor non-audio-visual session content, and removing jitter and adjustingPCR timing. The ITEM 50 creates an aggregate MPEG2 multiplex, (from thecontent and signaling of multiple sessions,) from any input SPTM to anyoutput MPTM. As is performed by the ITEM 30 in the hub 14, the ITEM 50uniquely reassigns PID values, creates an aggregate stream PSI andencrypts each SPTM. The ITEM 50 also updates the aggregate stream PCRand inserts ECMs. It also performs creates an aggregate multiplex of L1G20 signaling and addressable controller 24 messages, including EMMs, onthe OBTM which is then forwarded to the QPSK mux/mod 58. Each ITEM 50keeps track of the resources required for the active sessions. Shouldthe ASEM 22 need to recover its database, it queries the different ITEMs50 it oversees through the OAM&P interface (SNMPv2/UDP/Ethernet).

The single physical interface (OC-3c) into the ITEM 50 from the backbonesubnetwork 40 allows for rate policing of both session content andsignaling by the backbone subnetwork 40.

The QAM multiplexer modulator 52 accepts the downstream IBTM output ofITEM 50, adds signaling information in support of medium access control(MAC), inserts medium-access control synchronization information andapplies forward error correction and 64 QAM encoding. The output of theQAM mux/mod 52 is input to the RF upconverter 54.

Downstream OBTM signaling from the L1G 20 or the addressable controller24, (and optional application downloads from the L1G 20, or L1G 20 menusto VIPs 12), are sent from the ITEM 50 to the QPSK multiplexer modulator58. The QPSK mux/mod 58 accepts the output of the ITEM 50 andmultiplexes MAC information as in the in-band case. The QPSK mux/mod 58also provides forward error correction and QPSK modulation for the OBTMtransmission. The output is input to the RF combiner 59 (not shown) fordownstream transmission.

The primary function of the network controller 62 is to administermedium access control (MAC) functions for the network module 70interfacing to the network 10. MAC is a sublayer of the data link layerwhich coordinates the transmission of data from various network modules70 to the headend 18 based on fair access and predictability ofperformance. The network controller 62 determines the MAC operatingparameters based on those supplied by the ASEM 22 including but notexclusive to: 1) fiber to the node (FTTN) size; 2) upstream spectrumallocation (8-12 MHz, 8-15 MHz); 3) return path transmission rates; and4) QPSK mux/mod 58 and QPSK demux/mod 60 configuration (i.e.connectivity to the network controller 62 which will allow the networkcontroller 62 to route information such as acknowledgments to upstreamtransmitted packets), through the appropriate QPSK mux/mod 58 or QAMmux/mod 52.

The network controller 62 also administers the type of upstream accessthat will be required: 1) plain old polling (POP); 2) default assignedTDMA; and 3) dynamically assigned TDMA carriers (frequency and time slotassignments). The resources allocated by the network controller 62 forconnection-oriented MAC service requests are based upon the desiredsession bandwidth passed by the ASEM 22 on behalf of the L1G 20 or theVIP 12. Medium access control acknowledgment messages and informationare forwarded to the STTs 16 over Ethernet via the QAM mux/mod 52 andthe QPSK mux/mod 58.

The network controller 62 supports upstream access for interactiveservices requested from STTs 16 by: 1) administering the adaptive MACoperation to maintain guaranteed minimal upstream latency and ensurefair upstream access by all subscribers; and 2) forwarding upstream datato the L1G 20, L2G, or addressable controller 24 via the ATM servicesmux 64. The network controller 62 also allows for the dynamicprovisioning of multiple effective transmission rates according to theneeds of the applications and services utilizing the network 10.

The network controller 62 alleviates the L1G 20 from any functionspertaining to upstream access by overseeing network module 70initialization (default carrier, default time division multiple access(TDMA) transmission time slots, etc.) as well as overseeing dynamic TDMAcarrier and time slot assignment whenever a VIP-VIU session isestablished.

In the preferred embodiment, the network controller 62 is configured toaccommodate n ×(1.5 Mbps) streams from a QPSK demod/mux 60 (1<n<5) andforwards the upstream ATM cells to an appropriate externalrouting/switching element.

The network module 70 interfaces the STT 16 to the hybrid fiber coaxial(HFC) physical medium 56. The network module 70 facilitates theextraction of RF signals and demodulating (64QAMor QPSK) these signals.Forward error correction is then performed before any data link layer(DLL) processing takes place. As part of the DLL processing, the networkmodule 70 may decrypt certain IBTM or OBTM components and performAAL5-SDU based (upper layer) signaling extraction. It forwards AAL5-SDUprotocol data units (PDU) to the STT central processing unit as well asthe service stream to the other STT processing elements.

The network module 70, under the management of the network controller62, forwards signaling from the STT 16 to the corresponding networkcontroller 62 which forwards this information to the L1G 20 or to theVIP 12 through the backbone subnetwork 40. The network module 70 alsocommunicates with the addressable controller 24 for access control anddecryption/encryption authorization.

The QPSK demod/mux 60 receives up to six upstream carriers, demodulatesthe carriers and performs forward error correction. The resulting ATMcell streams are multiplexed to form a nominal 1.5 Mbps stream which isforwarded to the network controller 62. Additionally, the QPSK demod/mux60 forwards measured values of certain physical layer parameters to thenetwork controller 62 for network module 70 power calibration, andperforms upstream synchronization supporting functions to facilitateranging of STTs 16 (propagation delay calibration) for optimal TDMAoperation.

The components shown in FIG. 3 which have not been accompanied herein bya specific description operate as the equivalent components shown inFIG. 2.

Although the aforementioned description of the specific componentsprovides an understanding of the function of each component, a thoroughunderstanding of the network architecture of the present invention willbe further facilitated by a description of the signaling andcommunications between network components.

The information from a VIP 12 to a STT 16 flows on an IBTM which ismodulated as a 64QAM signal, while information from the L1G 20 to a STT16 flows through either an IBTM or an OBTM, modulated as a QPSK signal.A downstream path consists of both an IBTM and an OBTM destined to a STT16. An upstream signaling path consists of STT 16 to L1G 20, VIP 12,addressable controller 24, or network controller 62 signaling, via thenetwork module 70, through the network controller 62, and through thebackbone subnetwork 40. In addition, an upstream path consists ofsignaling between the network module 70 and the network controller 62.All upstream signals are QPSK modulated.

With respect to the downstream path IBTM, the CATV communication network10 uses the backbone subnetwork 40 to interconnect the VIP 12 to theITEMs 30, 50. Digital streams containing compressed video materialembedded in ATM cells originate at the VIP 12. AAL5 adaptation is usedto transport a MPEG2 SPTM over ATM. Additionally, signaling originatingfrom the VIP 12 to the STT 16 is carried as IP/ATM (or any protocol/ATM)using AAL5 adaptation as well. The ITEMs 30, 50 accept a plurality ofthe ATM streams, as instructed by the ASEM 22, to create an aggregateMPTM whose aggregate nominal rate is 27 Mbps. The VIP 12 informs the L1G20 of the service and signaling rates required in support of a givenSPTM. The L1G 20 in turn forwards this information to the ASEM 22. TheASEM 22 determines which of the ITEMs 30, 50 present within a givenheadend 18 can accommodate this SPTM. The ASEM 22 then conveys thisparameter along with other parameters such as MPEG2 program number to beassigned to the SPTM, ITEM 30, 50 output port (1 of 6). This mappingfrom ATM VCI to MPEG2 program number allows the reservation of thevirtual channel (connection) end-to-end through the ITEM 30, 50 (andterminated at the STT 16) whenever a new SPTM has to be multiplexed fordelivery to a single SST 16 or multiple STTs 16, (single STT 16 in thecase of an interactive service and multiple STTs 16 in the case of abroadcast service). The output of the ITEM 30, 50 consisting of a MPTMis then forwarded to the 64 QAM mux/mod 32, 52 and then to the RFupconverter 34, 54 to place the 64 QAM signal in the appropriate 6MHzchannel.

With respect to the downstream path OBTM, the output of the L1G 20signaling is delivered to the ITEM 30, 50 through the same 0C-3cinterface as the IBTM. Each ITEM 30, 50 has the capability of generatingan OBTM, carrying L1G 20 signaling for L1G-VIU default connections aswell as EMMs originating from the addressable controller 24. The OBTMoutput of the ITEM 30, 50 is forwarded to a QPSK mux/mod 36, 58.

With respect to the upstream path, the STT 16 central processing unitforwards signaling to the network module 70 which segments the data intoindividual ATM cells (AAL5) and forwards the cell based PDUs to theSTT's 16 QPSK modulator (not shown). Any non-VIP-VIU session signalingis sent over a default upstream carrier (such as that destined to theL1G 20 or network controller 62). All session-related signaling isforwarded on a dynamically assigned carrier which is assigned by thenetwork controller 62 during session connection establishment. The VIP12 specifies the effective session upstream signaling rate and thedownstream session signaling and content rate, depending on theactivated application, to the L1G 20. The L1G 20 in turn forwards theseparameters to the ASEM 22. The downstream rates are forwarded to theITEM 30, 50 while the upstream signaling rate is provided to the networkcontroller 62. A default upstream signaling rate is assumed by thenetwork controller 62 if the VIP 12 leaves this parameter unspecified.The ASEM 22 also forwards the upstream VPI/VCI pair to the networkcontroller 62 on VIU-VIP session by session basis, which in turn informsthe network module 70 during connection establishment phase. The networkmodule 70, as part of the initialization process, is aware of thedefault upstream carrier and the VPI/VCI values primarily used for STT16-L1G 20 signaling.

Each upstream carrier is demodulated by a QPSK demod/mux 60 which, inaddition to demodulating a given number of carriers, multiplexes theindividual upstream PDUs (preserving PDU boundary) and forwards themultiplex to the network controller 62. Using a monitoring means 88, thenetwork controller 62 examines each PDU and processes it by eitherforwarding it to the ATM services mux 64 (which performs an ATM relayfunction to the L1G 20 or the VIPs 12) or forwarding it to theappropriate processing element within the network controller 62 when thePDUs are MAC specific.

Referring to FIG. 4, the topology of the signal transmission network 56between the headend 18 and the STTs 16 is preferably a hybrid starfollowed by a tree and branch topology. In the predominantly star-typeaspect of the hybrid fiber-coax network, a fiber-optic line 90 isprovided from the headend 18 to each node 92. Each node 92 includesforward and return signal amplifiers and splitters/combiners to supportthe bi-directional communications. The single optical input 90 isconverted into an electrical signal which is split into four coaxialfeeder lines 94 which are generally dedicated to a limited group of STTs16 such as neighborhood. In the predominantly tree and branch aspect ofthe hybrid network, the feeder lines 94 connect the node 92 to thecoaxial drop lines 96 of individual STTs 16. At points where the cabledivides, signal splitters/combiners 29 are installed. Signal amplifiers98 may also be provided along the coaxial feeder lines 94 as required toboost the RF signals and ensure that nominal signal strength levels aremaintained.

Table 1 provides the I/O interfaces of the pertinent components in thepreferred embodiment.

                  TABLE 1                                                         ______________________________________                                        COMPONENT      INTERFACES                                                     ______________________________________                                        Addressable Controller                                                                       I/O:                                                                          2/10BaseT Ethernet                                                            32 × RS-232 (4 × multipin-                                        connector driving 8 port                                                      concentrator)                                                  Network Controller                                                                           Input:                                                         (NC 1000)      5 × EIA/RS-485 DTE I/F                                                  Outputs:                                                                      1 × ATM/DS3 (information rate                                           <=9 Mbps)                                                                     I/O:                                                                          2 × Ethernet 10BaseT                                     RF Modules     64QAM MUX/MOD                                                  Data Interfaces                                                                              Input:                                                                        1 × TAXI @ 27 Mbps                                                      Output:                                                                       1 × IF @ 43.75 MHz                                                      (75 Ohm F-Connector)                                                          I/O:                                                                          2 × Ethernet 10BaseT                                                    QPSK MUX/MOD:                                                                 Input:                                                                        1 × EIA/RS-530 @ 1. 5 Mbps                                              Output:                                                                       1 × RF, Range: 71-129                                                   MHz (75 Ohm F-Connector)                                                      I/O:                                                                          2 × Ethernet 10BaseT                                                    QPSK DEMOD/MUX:                                                               Input:                                                                        Upto 6 RF Inputs                                                              Output:                                                                       1 × EIA/RS-485 @ 1.5 Mbps                                               I/O:                                                                          2 × Ethernet 10Base T                                    Integrated Transport                                                                         Input:                                                         Encryption Mux (ITEM                                                                         1 × Optical Carrier 3-                                   1000) Data Interfaces                                                                        Concatenated (OC-3c, 155.52 Mbps)                                             Outputs:                                                                      5 × TAXI @ 27 Mbps                                                      1 × RS-530 @ 1.544 Mbps                                                 I/O:                                                                          1 × Ethernet 10Base T                                                   1 × RS-232 @ 19.2 Kbps                                   Network Module Input:                                                         Interfaces     RF via F Connector                                                            Output:                                                                       RF Bypass via F Connector                                                     Video via RCA phono plug                                                      Audio Right and Left Channels                                                 via RCA phono plug                                                            I/O:                                                                          Digital bidirectional                                                         interface, (32-pin molex                                                      connector)                                                     ______________________________________                                    

Referring to FIG. 5, a block diagram of a SST 16 is shown.Communications in the downstream path originating at the headend 18 aretransmitted through the coaxial drop line 96. Video information signalsare processed through a frequency agile tuner and a descrambler timingand control module (optional--if analog video is scrambled) 112 beforeentering a television set 114. The tuner 110 is responsive to thefrequency of the downstream channel selected by the VIU to remove thecarrier signal. The descrambler 112 descrambles the baseband signal ofthe selected channel if the VIU is an authorized user. Similarly,digital video passes through the decryption and then MPEG2 decodes andD/A conversion to forward the composite video for display. (The basebandvideo signal is placed on a second carrier signal frequency, typicallytelevision channel 3 or 4, for input into the television 114). The tuner110 and descrambler 112 are controlled by the network module 70, whichincludes an encryption/decryption controller 124 and a MAC module 126.

The network module 70 interfaces with a processor 116 which comprises acentral processing unit (CPU) 118, a read only memory (ROM) 120 and anon-volatile random access memory (RAM) 122. Several processingcomponents reside in the RAM 122, including an existing protocol syntaxprocessor 128 (PSP), an adaptive protocol processor 130 (APP), a memorymanager 132 and a transmit queue 134. The remaining portion of the RAM122 is available for processing higher layers of the protocol and forgeneral use. The APP 130, which includes a primitive PSP, is alsoresident in ROM 120 for initial start-up and recovery operation. The CPU118, in conjunction with the different processing components in RAM 122,allows the STT 16 to read incoming data frames, parse the frames bysequentially stripping off each nested layer, and perform theapplication embedded therein.

Data frames within the OBTM forwarded through the headend 18 to the STT16 are received through a second frequency agile receiver 140, which istuned to the particular control channel for receiving control messages.In the preferred embodiment, the transmitter 142 transmitscommunications upstream from the STT 16 to the network controller 62.

The STT 16 is controlled via the subscriber remote control 144. Theremote control 144 has an infrared (IR) signal emitter 146 which sendsIR control signals to the IR receiver 148 as is well known in the art.The received control signals are then forwarded to the processor 116.

The preferred embodiment of the present invention employs a hybridmedium access control (MAC) system 400 to control access to the upstreambandwidth by the plurality of STTs 16. The hybrid MAC system 400comprises different MAC components which are selected based upon thetype of communication that is associated with the correspondingapplication or service selected by the VIU. Each MAC component resideson a separate frequency. MAC parameters are configurable to provideadditional flexibility and operational and resource optimization (suchas latency, probability of blocking and bandwidth utilization).

Applications and services selected by the VIU can be categorizedaccording to the communications required to support the application orservice. The first category of applications and services are associatedwith asynchronous, latency independent communications. Thesecommunications are the least demanding from a performance standpointsince the applications and services are capable of functioningeffectively with response time latencies of minutes or hours. This istypical of transactions that do not require a subsequent action to beperformed by the application or the network 10 in support of a VIUvisible response to an original action. For these applications andservices, the time that it takes to deliver the data to the associateddestination is not a critical factor.

The second category of applications and services are associated withasynchronous, contention-prone communications. These communicationsplace medium demands on performance and thus, maximum response latencieson the order of sub-seconds are required. This category includestransactions and services that are followed by a subsequent action atthe application, the L1G 20, or the VIP 12 in support of a VIU visibleresponse to an original action. Timely delivery of information on theorder of microseconds is critical for applications such as informationon demand and video-on-demand applications.

The third category of applications are associated with isochronouscommunications. These are the most demanding in terms of bandwidth,guaranteed maximum latencies (milliseconds), and a symmetric utilizationof upstream and downstream bandwidths. Advanced multi-mediaapplications, plain old telephony service (POTS), personal communicationservices (PCS), and video telephony are some of the applications in thiscategory. By allocating a separate portion of the bandwidth fordifferent types of communications, the hybrid MAC system 400 of thepresent invention will support future services that are not yetenvisioned.

Referring to FIG. 7, the configurable hybrid MAC system 400 employsspace division multiplexing, frequency division multiplexing, and timedivision multiplexing to efficiently utilize the upstream bandwidth. Thespace domain allows for dividing the subscriber population into smallersegments based on a number of factors such as physical location,subscriber population density and projected growth. Each networkcontroller 62 may be assigned a number of physical signal transmissionpaths, such as a fiber 90 and associated carriers. This allows forgradual network expansion from the physical topology standpoint sinceadditional network controllers 62 may be added to the headend 18 basedupon the performance needs of the network which are a function of fiberto the node (FTTN) size, service take rate, or simultaneous usage of agiven service, etc.

The frequency domain using frequency division multiplexers 57,facilitates the efficient support of the hybrid MAC components as wellas further segmentation of the STT 16 population. Each of the hybrid MACcomponents are allocated a portion of the upstream bandwidth having atleast one RF carrier frequency. This is a primary factor behind thesimplicity of the design that could not be achieved by prior art MACmethods that attempted to support asynchronous, anisochronous andisochronous data without distinct channel separation.

The time domain using time division multiplexers 59, is used to allowmultiple access at the smallest granularity as compared to the other twodomains. The time domain is the basis for the fixed and both types ofdynamic TDMA MAC components, (dynamic multi-rate TDMA and random slotreservation-dynamic slot assignment (RSR-DSA) TDMA). Multiple accesswithin the same STT 16 population segment and the same allocatedupstream frequency is achieved in this domain.

The hybrid MAC system 400 is adjustable to the variety of current andfuture applications that may be added from a network resource supportstandpoint by categorizing each application (as aforementioned)according to the network resources required to support the followingtypes of communications: 1) isochronous communications; and 2)asynchronous communications including, latency independent andcontention-prone communications.

In operation, the network controller 62 receives via a receiver 87, aplurality of service requests over preassigned default connections fromthe plurality of STTs 16. These requests are forwarded to the networkcontroller 62 through the QPSK demod/mux 60. Once allocation ofresources (frequency and time domain) are selected by the processer 85confirmation is sent downstream to the MAC module 126 via OBTM throughan appropriate QPSK mux/mod 58. The process is repeated by other STTs 16as application and service session requests are made.

Referring again to FIG. 5, the preferred STT 16 for implementing thehybrid MAC system 400 is shown. The STT 16 includes a medium accesscontrol module 126 and a decryption/encryption control module 124 forcommunicating with the network controller 62 through the transmissionnetwork 56. The STT 16 initiates a service request when the VIU selectsa particular application, service, or session. The MAC module 126analyzes, based on well known application service types, thecommunication requirements (i.e. isochronous, asynchronous) of therequested service or session type, and selects the MAC component whichwill most closely meet the communication requirements. Since each MACcomponent has a preassigned upstream bandwidth, the frequency agile datatransmitter 142, at the direction of the MAC module 126, is tuned to afrequency allocated to the particular MAC component. The STT 16thereafter communicates over that frequency until the connection isreleased (communication terminates) and/or the network controller 62reassigns or reallocates resources.

Alternatively, a network controller client, such as ASEM 22, may request(on behalf the L1G) the allocation of resources in support a session ofa given service. Once the processer 85 within the network controller 62,allocates the appropriate MAC resources, it informs the STT 16, via atransmitter 86 of the relevant MAC parameters associated with theconnection established in support of the requested session. A detailedblock diagram of a network controller 62 which implements MAC is shownin FIG. 10.

Referring to FIG. 6, a TDMA frame 410 as used by the assigned dynamicmulti-rate TDMA MAC component is shown. The frame 410 comprises a seriesof time slots 412 which may be separated in time 414. The configurableparameters for the assigned dynamic multi-rate MAC component are theframe size, the time slot size and the spacing 414 between time slots412. The size of the frame 410 may be varied by increasing or decreasingthe number of time slots 412. This affects the latency and the effectivetransmission rate of the communication. The size of the time slot 412may also be varied to change the number of clock cycles within a timeslot, and therefore the number of packets, that may be transmitted in agiven time slot 412 (again, changing the effective transmission rate).The spacing 414 between time slots 412 may be varied to change thenumber of packets which may be transmitted within a given frame 410. Theadjustment of the spacing 414 between time slots 412 is also used forpropagation delays to compensate for the different physical distances ofthe STTs 16 from the network controller 62.

The static TDMA MAC component is typically assigned by the networkcontroller 62 based on a connect request from a network controllerclient such as the ASEM 22. For the static TDMA MAC component, the TDMAframe is fixed at one time slot per STT 16. This provides guaranteedbandwidth for access by STTs 16 within a given frame time conveyingadditional connection requests or conveying diagnostic/statusinformation which may also be forwarded on the POP carrier).

In the preferred embodiment, the RSR-DSAMAC component is the defaultTDMA channel for large nodes with high take rates in place of the staticTDMA. If the MAC module 126 has determined that the requested service issuited for random slot reservation--dynamic slot allocation (RSR-DSA),the STT 16 initiates upstream communications by randomly selecting atime slot 412 within a given frame 410 or by transmitting on apreviously reserved time slot 412. The probability of collisions isinversely proportional to the number of time slots 412 per frame 410 anddirectly proportional to the number of STTs 16 that need to communicateduring that cycle. The network controller 62 assigns the RSR-DSA TDMAframes 410 in accordance with the procedure shown in FIG. 8. In step 300the STT 16 may reserve a time slot 412 once, or may request reservingthe time slot 412 over multiple cycles. This reservation request istransmitted upstream over a randomly selected time slot 412 (step 302).If a collision is detected by the network controller 62, the networkcontroller 62 transmits a message to the STTs 16 to retransmit therequest (step 303). If no collision is detected by the networkcontroller 62, the network controller 62 receives the request (step 304)and sends an acknowledgment to the requesting STT 16 and to all otherSTTs 16 that the particular slot within that channel is no longeravailable (step 306). The STT 16 receives this acknowledgment (step 308)and the requesting STT 16 beings communications. The network controller62 dynamically assigns the requested time slot 412 by acknowledging areservation by a message sent to the STT 16 in the downstream OBTM.After the STT 16 has terminated communications (step 310), the time slot412 is released. The network controller 62 monitors the channel activity(step 312), and when it determines (step 314) that the time slot 412 hasbeen released, a "time slot free" message is sent to all STTs 16 (step316.) All STTs 16 receive an acknowledgment that the time slot 412 isavailable (step 318). If, while monitoring channel activity (step 412),the network controller 62 determines that there is too much activityover a particular channel (step 320), the network controller 62 adjuststhe number of time slots 412, and increases the size of the frame 410,or allocates additional frequencies (step 322). Transmission efficiencymay also be increased by performing ranging which accounts forpropagation delays. This adjusts the spacing 414 between time slots 412thereby allowing for additional time slots 412. If the increased framesize is greater than the corresponding fixed TDMA frame size, (step 324)the network controller 62 determines that particular channel is to bedesignated a static TDMA channel (step 326).

While the RSR-DSA describes the connectionless, mode of operationintended for the default TDMA channel in a large node size and hightake-rate, the connection-oriented mode is similar in the sense that atime slot 412 or set of time slots 412 may be reserved over multiplecycles (frames) for the duration of the connection. Additionalinformation may be conveyed by the network controller 62 duringconnection establishment and connection release phases.

In the connection-oriented mode, where assigned dynamic multi-rate TDMAis used, the network controller 62 may assign a frequency and a timeslot(s) 412 for the duration of the connection, (typical of applicationsbenefiting from guaranteed bandwidth hence predictable latency insupport of interactive applications). Referring to FIG. 9, in this mode,a network controller client (e.g. ASEM 22), or a network module 70client (e.g. an application within the STT 16) forwards a connectionrequest (step 700) to the network controller 62 specifying which STT 16is to be connected and the associated TDMA rate desired for thatconnection, (480 bps, . . . 2400 bps . . . 32 kbps . . . 142 kbps,11520, 16000, 19200, 15360, 32000, 56000, 64000, 112000, 128000, 192000bps). This rate is a function of the level of interactivity which ischaracteristic of the application. For isochronous services (video,telephony or POTs), the rates depend on the service itself (e.g. video,telephony using 64 kbps or 128 kbps).

The network controller 62 checks the available resources (step 702) todetermine if the new request can be accommodated. If required resources(e.g. number of time slots 412) are not available (on any of thefrequencies supporting the requested rate or lower multiples of thatrate), the network controller 62 rejects the connection requestspecifying the reason (e.g. no available resources). If on the otherhand the network controller 62 determines that the required resourcesare available (step 704) it reserves the selected time slots 412 withinthe appropriate frequency (step 708), informs the network module of suchparameters, and returns to the client a confirmation to the connectionrequest (step 710).

When the connection request is originated by ASEM 22, the request isforwarded on the network controller's 62 Ethernet port. When the requestis forwarded by the STT 16, the network module 70 may forward therequest on the default static TDMA channel or the RSR-DSA default TDMAchannel (whichever is employed at the time within a given system).

The plain old polling component of the MAC system 400 is intended forapplications that benefit from a store and forward system wherecollisions are expected but the latency associated with POP response isirrelevant. Additionally, it facilitates controlled communications forany diagnostic operation and presents a fall back communication methodfor diagnostic purposes should other components of the hybrid MAC system400 fail.

If the MAC module 126 determines that POP is required, the datatransmitter 142 is tuned to the POP frequency and a service message isplaced in the transmit queue 134.

The polling service may be initiated periodically by the networkcontroller 62 to instruct the STT 16 to purge their transmit queue 134,or a polling request may be transmitted by the STT 16. The transmitqueue 134 operates as a first-in-first-out (FIFO) buffer which residesin the settop RAM 122. When an application or service requires to send amessage upstream, the information is forwarded utilizing resourcesassigned to the connection supporting the application session. If theinformation is in response to a poll, the message is copied into thetransmit queue 134.

The STT 16 transmits the FIFO entries when it receives the pollingtoken, which is an instruction by the network controller 62 for the STT16 to empty its transmit queue 134. The polling token is broadcast onall OBTM downstream channels and the STT 16 transmits on the pollingchannel frequency. When the network controller 62 carries out thepolling cycle, it has no knowledge of the OBTM downstream frequency towhich the STT 16 is tuned. This necessitates sending the polling tokenon all downstream OBTM carriers for a given neighborhood. Once a STT 16gets the token, it is permitted to empty its entire transmit queue 134.The objective is to minimize the number of individual polling cyclesrequired to empty the STT's entire queue 134. The network controller 62then reads the messages from the plurality of STTs 16.

In the preferred embodiment, each node 92 provides service to fivehundred STTs 16. In this configuration, at least one upstream defaultchannel is designated assigned static TDMA mode; at least one upstreamdefault channel is allocated to the POP communicating node; and theremaining upstream channels are assigned dynamic multi-rate TDMA.RSR-DSA mode channels are allocated upon request. In any givenimplementation, the hybrid MAC components may reside on a singlefrequency. For example, a channel that has been designated as a TDMAchannel may have a set of time slots 412 that belong to the static TDMAoperation and another set that belong to the dynamic TDMA operation.This, however, leads to a more complex implementation within the networkcontroller 62.

The hybrid MAC of the present invention supports isochronous andanisochronous multi-media communications data in a cost effective andefficient manner. In addition, connection-orientated as well asconnectionless services have been supported, where the former requires aguaranteed bandwidth allotment over the duration of the connection.

Although the invention has been described in part by making detailedreference to certain specific embodiments, such details is intended tobe instructive rather than restrictive. It will be appreciated by thoseskilled in the art that many variations may be made in the structure andmode of operation without departing from the spirit and scope of theinvention as disclosed in the teachings herein.

I claim:
 1. A system for controlling access to a bi-directional cabletelevision hybrid fiber-coax network having a plurality of upstream anddownstream communication channels, each upstream channel supporting adifferent communication mode, the system comprising a headend unit andat least one subscriber unit:said subscriber unit having:means forreceiving a request for transmitting over said network; means fordetermining the type of communication to be transmitted; means forselecting a communication mode, based upon said determination, fortransmitting said communication; means for transmitting a connectionrequest over one of said upstream channels; means for receiving aconnection acknowledgment over one of said downstream channels, whereinsaid acknowledgement includes an authorized communication channelassignment; and means for transmitting communications over saidauthorized channel; and said headend unit having:means for receivingsaid connection request from said subscriber unit; means for monitoringcommunication traffic over an upstream channel associated with saidselected communication mode; means for configuring selected parametersof said communication mode, said configuring means being responsive tosaid monitoring means; and means for transmitting a connectionacknowledgment over said downstream channel to said subscriber unit. 2.The system of claim 1 wherein said upstream channels are further dividedinto repeating frames, each frame having a plurality of time slots andsaid headend unit further includes means for notifying said subscriberunit when time slots are available for communicating.
 3. The system ofclaim 2 wherein said communication modes include plain old polling;assigned static TDMA; assigned dynamic multi-rate TDMA; and random slotreservation--dynamic slot allocation TDMA.
 4. The system of claim 3wherein said selected parameters include time slot size, frame size,number of time slots and spacing between time slots within saidcommunication mode.
 5. The system of claim 2 wherein each of saidcommunication modes is supported by a plurality of upstreams channels,and said headend unit further includes means for dynamically allocatingsaid channels among said communication modes.
 6. The system of claim 5wherein said allocating means is responsive to said monitoring means. 7.The system of claim 2 wherein said headend is responsive to connectionrequests from communicating entities located upstream of said headend.8. A system for controlling access to a bi-directional informationtransport medium between at least one subscriber unit and a headendunit, the transport medium having a plurality of communication channels,each channel supporting a different communication mode, the systemcomprising:said subscriber unit having:means for receiving a request fortransmitting over said transport medium; means for determining the typeof communication to be transmitted; means for selecting a communicationmode, based upon said determination, for transmitting saidcommunication; means for transmitting communications over a selectedchannel associated with said communication mode; and means for receivingcommunications over a second channel; and said headend unit having:meansfor receiving said communications over said selected channel from saidsubscriber unit; means for monitoring communication traffic over saidchannel; means for configuring selected parameters of said communicatingmode, said configuring means being responsive to said monitoring means;and means for transmitting communications to said subscriber unit oversaid second channel.
 9. A system for controlling access to abi-directional information transport medium between at least onesubscriber unit and a headend unit, the transport medium having aplurality of communication channels, each channel supporting a differentcommunication mode, the system comprising:said subscriber unithaving:means for receiving a request for transmitting over saidtransport medium; means for determining the type of communication to betransmitted; means for selecting a communication mode, based upon saiddetermination, for transmitting said communication, wherein saidcommunication modes include plain old polling; assigned static TDMA;assigned dynamic multi-rate TDMA; and random slot reservation-dynamicslot allocation TDMA; means for transmitting communications over aselected channel associated with said communication mode; and means forreceiving communications over a second channel; and said headend unithaving:means for receiving said communications over said selectedchannel from said subscriber unit; means for monitoring communicationtraffic over said channel; means for configuring selected parameters ofsaid communicating mode, said configuring means being responsive to saidmonitoring means; andmeans for transmitting communications to saidsubscriber unit over said second channel.
 10. The system of claim 9wherein said selected parameters include time slot size, frame size,number of time slots and spacing between time slots within said dynamicmulti-rate TDMA communication mode.
 11. The system of claim 8 whereinsaid selected parameters include time slot size, frame size, number oftime slots and spacing between time slots.
 12. The system of claim 9wherein said headend further includes means for notifying saidsubscriber unit when a time slot within a channel has been released. 13.The system of claim 8 further including a plurality of subscriber units,wherein said subscriber units are divided into spatially diverse groups.14. The system of claim 8 wherein said subscriber unit further includesmeans for tuning a transmitter for transmitting over said selectedcommunication channel, and means for tuning a receiver for receivingover said second channel.
 15. The system of claim 9 wherein each of saidcommunication modes is supported by a plurality of channels, and saidheadend unit further includes means for dynamically allocating saidchannels among said communication modes.
 16. The system of claim 15wherein said allocating means is responsive to said monitoring means.17. The system of claim 9 wherein said headend is responsive toconnection requests from communicating entities located upstream of saidheadend.
 18. The system of claim 8 wherein said medium comprises ahybrid fiber-coaxial transmission system.
 19. The system of claim 9wherein said medium comprises a fiber optic transmission system.
 20. Thesystem of claim 8 wherein said medium comprises a coaxial transmissionsystem.
 21. A subscriber unit for controlling access to a bi-directionalinformation transport medium, the transport medium having a plurality ofcommunication channels, each channel supporting a differentcommunication mode, said subscriber unit comprising:means for receivinga request for transmitting over said transport medium; means fordetermining the type of communication to be transmitted; means forselecting a communication mode, based upon said determination, fortransmitting said communication; means for transmitting communicationsover a selected channel associated with said communication mode; andmeans for receiving communications over a second channel.
 22. A headendunit for controlling access to a bi-directional information transportmedium by a plurality of subscriber units, the transport medium having aplurality of communication channels, each channel supporting a differentcommunication mode, said headend unit comprising:means for receivingconnection requests over one of said channels from a plurality ofsubscriber units; means for monitoring communication traffic over saidchannels; means for allocating said plurality of channels among saidcommunication modes, said allocating means being responsive to saidmonitoring means; means for configuring selected parameters of saidcommunication modes, said configuring means being responsive to saidmonitoring means; and means for transmitting connection authorizationsto said subscriber units over a second channel.
 23. A method forcontrolling access to an information transport medium, the transportmedium having a plurality of communication channels, each channelsupporting a different communication mode, the methodcomprising:receiving a request for transmitting over said transportmedium; determining the type of communication to be transmitted;selecting a communication mode, based upon said determination, fortransmitting said communication; transmitting a connection request forcommunicating over said selected communication mode; receiving aconnection acknowledgment, comprising an authorized communicationchannel; and transmitting said communication over said authorizedchannel.
 24. A system for controlling access to a bi-directional cabletelevision hybrid fiber-coax network having a plurality of upstream anddownstream communication channels, each upstream channel supporting adifferent communication mode, the system comprising a headend unit, atleast one network element manager located upstream of said headend unitand at least one subscriber unit located downstream of said headendunit:said network element manager having:means for specifying connectionparameters for transmitting a communication over said network, saidconnection parameters including a connection rate; means fortransmitting said connection parameters over one of said downstreamchannels; means for receiving a connection acknowledgment over one ofsaid upstream channels; and means for transmitting communications overone of said downstream channels; and said headend unit having:means forreceiving said connection parameters from said network element manager;means for determining the communication mode based upon said connectionparameters; means for monitoring communication traffic load over aplurality of upstream channels; means for configuring said mode, saidconfiguring means being responsive to said monitoring means; and meansfor transmitting a connection acknowledgment over said upstream channelto said network element manager.
 25. The system of claim 24 wherein saidconfiguring means includes means for determining medium access control(MAC) parameters including upstream channel frequency, frame size, timeslot size and spacing between time slots.
 26. The system of claim 24wherein said subscriber unit includes:means for receiving a connectionrequest from the headend unit for transmitting over said network, saidrequest including said selected communication mode and said connectionparameters; and means for transmitting communications in accordance withsaid MAC parameters; andsaid headend unit further includes: means fortransmitting communications over one of said downstream channels to saidsubscriber unit; and means for receiving communications over one of saidupstream channels from said subscriber unit.
 27. A method forcontrolling access to an information transport medium, the transportmedium having a plurality of communication channels, each channelcapable of supporting at least one communication mode, the methodcomprising:receiving a request for transmitting over said transportmedium; determining the type of communication to be transmitted;selecting a communication mode, based upon said determination, fortransmitting said communication; transmitting a connection request forcommunicating using said selected communication mode; receiving aconnection acknowledgment, comprising an authorized communicationchannel; and transmitting said communication over said authorizedchannel.
 28. The method of claim 27 wherein each communication mode isutilized on a plurality of channels.
 29. A system for controlling accessto a bi-directional information transport network between at least onesubscriber unit and a headend unit, the transport medium having aplurality of communication channels, each channel supporting at leasttwo communication modes, the system comprising:said subscriber unithaving:means for receiving a request for transmitting over saidtransport network; means for determining the type of communication to betransmitted based upon said request; means for selecting a communicationmode and an upstream channel, based upon said determination, fortransmitting said communication, wherein said communication modesinclude plain old polling; assigned static TDMA; assigned dynamicmulti-rate TDMA; and random slot reservation-dynamic slot allocationTDMA; means for transmitting communications over said selected upstreamchannel; and means for receiving communications over a downstreamchannel; and said headend unit having:means for receiving saidcommunications over said selected upstream channel from said subscriberunit; means for monitoring communication traffic over a plurality ofupstream channels; means for configuring selected parameters of saidcommunication mode, said configuring means being responsive to saidmonitoring means; and means for transmitting communications to saidsubscriber unit over a downstream channel.
 30. A system for controllingaccess to a bi-directional cable television hybrid fiber-coax networkhaving a plurality of upstream and downstream communication channels,each upstream channel supporting a different communication mode, thesystem comprising a headend unit, at least one network element managerlocated upstream of said headend unit and at least one subscriber unitlocated downstream of said headend unit:said network element managerhaving:means for specifying a type of communication for transmittingover said network; means for transmitting said communication type overone of said downstream channels; means for receiving a connectionacknowledgment over one of said upstream channels; and said headend unithaving:means for receiving said communication type from said networkelement manager; means for monitoring communication traffic load over anupstream channel associated with said communication type; means forselecting a communication mode and for configuring associated parametersof said mode based upon said communication type, said configuring meansbeing responsive to said monitoring means; and means for transmitting aconnection acknowledgment over an upstream channel to said networkelement manager.
 31. The system of claim 30 wherein said communicationtype includes synchronous, asynchronous, isochronous and anisochronouscommunications.
 32. The system of claim 30 wherein said subscriber unitincludes:means for receiving a connection request from the headend unitfor transmitting over said network, said request including thecommunication mode and associated parameters; means for transmittingcommunications in accordance with said parameters; andsaid headend unitfurther includes: means for transmitting said connection acknowledgmentover a downstream channel to said subscriber unit; and means forreceiving communications from said subscriber unit.